In a condenser microphone, an FET (field-effect transistor) or a vacuum tube is used as an impedance converter. This is because a condenser microphone capsule including a diaphragm and a fixed pole opposed to each other has quite a high impedance.
When a vacuum tube is used as the impedance converter of a condenser microphone, generally a dedicated power source is necessary. Further, “A” power source for heating the heater of the vacuum tube and “B” power source for passing current through the plate of the vacuum tube are necessary to operate the vacuum tube.
In general, a vacuum tube used as an impedance converter requires “A” power source of about 6.3 V and 0.35 A and “B” power source of about 120 V and 10 mA. Thus, a typical phantom power source of DC 48 V cannot afford such a power and thus a dedicated power source is used.
Incidentally, there is a vacuum tube which has low consumption and operated by batteries (e.g., a directly heated pentode, model 6418, Raytheon, US). This vacuum tube with low power consumption is developed exclusively for hearing aids and has “A” power source operating at 1.25 V and 10 mA and “B” power source operating at 30V and 0.24 mA.
The vacuum tube operating at such a voltage can be sufficiently operated by the phantom power source of DC 48 V. However, when the “A” power source (1.25 V, 10 mA) is stabilized as it is by a Zener diode or the like, a current of 10 mA passes through the heater of the vacuum tube, and thus a voltage supplied from the phantom power source decreases.
For example, when a current of 10 mA passes through the heater of the vacuum tube, a voltage supplied from the phantom power source to a microphone is reduced by 34 V (5 mA×6.8 kΩ) to 14 V. In this state, the “B” power source is insufficiently supplied to the plate of the vacuum tube, and thus this state interferes with the operations of the vacuum tube.
In order to enable a microphone using a vacuum tube as an impedance converter to be operated by a phantom power source, the present applicant applied Japanese Patent Application No. 2003-6621 (Japanese Patent Application Publication No. 2004-221919) before. In this invention, “A” power source for heating the heater of a vacuum tube is obtained from a phantom power source by using a down converter which causes little noise and is readily available.
The configuration of the condenser microphone according to the invention of the prior application will be schematically described with reference to the circuit diagram of FIG. 2. The microphone comprises a vacuum tube V01 as an impedance converter of a condenser microphone capsule MC. For example, the vacuum tube V01 is a directly heated pentode, model 6418, Raytheon, US.
In the vacuum tube V01, “A” power source operates at 1.25 V and 10 mA and “B” power source operates at 30 V and 0.24 mA. Reference numeral R01 denotes a bias resistor of the control grid of the vacuum tube V01, and reference numeral C01 denotes a feedback condenser from the plate to the control grid of the vacuum tube V01.
The plate of the vacuum tube V01 is connected to one end of the primary winding of an output transformer TRS, and the screen grid of the vacuum tube V01 is connected to the other end of the primary winding of the output transformer TRS via a voltage divider circuit including voltage divider resistors R02 and R03. The secondary winding of the output transformer TRS is connected to an output connector CN.
The output connector CN is, e.g., a 3-pin output connector defined by EIAJ RC5236. The output connector CN includes a terminal H connected to the hot side of a phantom power source (not shown), a terminal C connected to the cold side of the phantom power source, and a terminal G connected to the ground side of the phantom power source. The secondary winding of the output transformer TRS is connected to the terminal H and the terminal C. The ground side of the microphone capsule MC and the ground pole of the heater of the vacuum tube V01 or the like are connected to the terminal G.
The invention of the prior application comprises an “A” power source generating circuit 10 for obtaining “A” power source for the vacuum tube V01 from the phantom power source. The “A” power source generating circuit 10 includes at least one switched capacitor voltage converter IC and is connected to the heater of the vacuum tube V01. In the example of FIG. 2, three converters IC01, IC02, and IC03 (LM2665, National Semiconductor, US) are connected in series in three stages.
The converter IC, LM2665, has six pins. When a positive voltage is inputted from a first pin, the converter IC acts as a dual boost converter in which a doubled voltage is outputted from a fifth pin. Conversely, when a positive voltage is inputted from the fifth pin, the converter IC acts as a ½ step-down converter in which a half voltage is outputted from the first pin.
In the invention of the prior application, the converters IC01 to IC03 are used as ½ step-down converters. Thus, a positive voltage is inputted from the fifth pin and a half output voltage is obtained from the first pin in each of the converters IC01 to IC03. A second pin acts as a ground pin and a fourth pin acts as a shut-down control pin. Charge pump condensers C06, C08, and C10 are connected between third pins and sixth pins of the condensers IC01, IC02, and IC03, respectively. Condensers C05, C07, and C09 for smoothing and AC grounding are connected to the first pins of the output sides of the condensers IC01, IC02, and IC03, respectively.
The “A” power source generating circuit 10 comprises an input circuit 10a composed of a series circuit of a current limiting resistor R04 and a Zener diode D01, and constant current diodes D02 and D03 for drawing, through the input circuit 10a, a predetermined current supplied from the phantom power source through the terminals H and C.
For example, 30 V and 1.5 mA are supplied from the constant current diodes D02 and D03 to the input circuit 10a, an input voltage to the “A” power source generating circuit 10 is set to 10 V by the Zener diode D01 in the input circuit 10a, and an input current to the “A” power source generating circuit 10 is limited to 1.25 mA by the current limiting resistor R04.
Hence, in the converter IC03 of the first stage, the converter IC02 of the second stage, and the converter IC01 of the third stage, a voltage is sequentially changed to 5 V, 2.5 V, and 1.25 V and a current is sequentially changed to 2.5 mA, 5.0 mA, and 10 mA. In the end, the “A” power supply of 1.25 V and 10 mA is supplied from the converter IC01 of the third stage to the heater of the vacuum tube V01. In FIG. 2, reference numerals C02, C03, C04, and C11 denote smoothing condensers.
As described above, about 1.25 mA is enough as a current drawn from the phantom power source to the “A” power source generating circuit 10, and thus about 4.25 V is enough as a voltage drop for heating the heater. Hence, it is possible to sufficiently obtain “B” power source for applying current to the plate of the vacuum tube V01.
According to the invention of the prior application, although the condenser microphone having the vacuum tube used as the impedance converter can be operated by the phantom power source, a power source voltage is low and thus the “B” power source has an amplifier circuit configuration, in which the primary side of the output transformer TRS is connected to the plate of the vacuum tube V01.
Therefore, DC current to be supplied to the plate of the vacuum tube V01 passes through the primary side of the output transformer TRS, so that the core of the transformer is DC magnetized. When the DC magnetization increases, the core is magnetically saturated and the performance of the transformer considerably decreases. Thus, at worst, a voice signal may not be transmitted from the primary side to the secondary side.
As a method for preventing DC magnetization of a transformer core, the following methods are known: a method of increasing the size of a core; a method of using a material of low magnetic permeability for a core; and a method of positively forming a gap between cores so as to have magnetic leakage on the connection of the cores. In any case, the dimensions of the transformer increase and the performance undeniably decreases as compared with a transformer designed so as not to pass current.